Modern, broadband mobile radio systems use not only phase modulation but also an amplitude component for modulation in order to achieve a wider bandwidth, in order to make it possible to comply with the relatively stringent requirements for data transmission rates.
Linear modulation methods of this type, such as those envisaged in the GSM EDGE (Global System for Mobile Communication) Standard, place more stringent requirements on the linearity characteristics of transmitting and receiving arrangements in mobile radio and on carrier and side band suppression at the output of a transmitting arrangement, however. These are in turn dependent on manufacturing tolerances, in particular matching tolerances between the components used in a transmitting arrangement.
Quadrature modulators in transmitting arrangements for mobile radio to which, for example, a complex baseband signal can be supplied, which is modulated onto a carrier frequency, require a local oscillator signal which likewise comprises two components shifted through 90° with respect to one another. Discrepancies from signal orthogonality, that is to say from the phase shift of 90° for the carrier signal, lead, for example owing to excessively large matching tolerances between the components in an upstream frequency divider, to inadequate side band suppression in the frequency mixer and/or in the IQ modulator.
Furthermore, offset voltages in the baseband signal (which may arise, for example, because of mismatches between the components that are used) cause inadequate carrier suppression in the mobile radio transmitter. The carrier suppression may also be made worse as a result of inadequate isolation between the local oscillator signal input and the signal output of the modulator, causing crosstalk from the carrier signal to the modulator output. However, inadequate carrier and side band suppression results in a phase error in the digitally modulated output signal of the transmitting arrangement.
The document by M. Faulkner, T. Mattsson, and W. Yates entitled “Automatic Adjustment of Quadrature Modulators”, Electronics Letters, Jan. 31st, 1991, Vol. 27 No. 3, pp. 214-216, describes a quadrature modulator with a feedback path which acts on a correction network at the input of the modulator in order to compensate for phase errors as well as offset errors in the transmitting arrangement by appropriate preemphasis of the baseband signal. In this case, however, a directional coupler for outputting the radio-frequency transmission signal, an additional amplifier as well as a downstream power detector are required in the feedback path which is connected to the output of the modulator, and these result in a relatively large chip area requirement, additional, undesirable power consumption, and increased manufacturing costs.
An object of the present invention is to specify a transmitting arrangement, in particular for mobile radio, which can be produced with little complexity and is suitable for mass production, while maintaining the carrier and side band suppression that are required for linear modulation methods.